UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


DEHYDRATION  OF  FRUITS 

(A  PROGRESS  REPORT) 

BY 

W.  V.  CRUESS  and  A.  W.  CHRISTIE 


BULLETIN  No.  330 

September,  1921 


UNIVERSITY  OF  CALIFORNIA  PRESS 
BERKELEY,  CALIFORNIA 


David  P.  Barrows,  President  of  the  University. 


EXPERIMENT  STATION  STAFF 

HEADS  OF  DIVISIONS 

Thomas  Forsyth  Hunt,  Dean. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

,  Director  of  Eesident  Instruction. 

Clarence  M.  Haring,  Veterinary  Science,  Director  Agricultural  Experiment 
Station. 

B.  H.  Crocheron,  Director  of  Agricultural  Extension. 

James    T.    Barrett,    Acting   Director    of     Citrus    Experiment    Station,    Plant 

Pathology. 
William  A.  Setchell,  Botany. 
Myer  E.  Jaffa,  Nutrition. 
Ralph  E.  Smith,  Plant  Pathology. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 
John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Fruit  Products. 
Warren  T.  Clarke,  Agricultural  Extension. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
Walter  Mulford,  Forestry. 
Fritz  W.  Woll,  Animal  Nutrition. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  Quayle,  Entomology. 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology. 
J.  C.  Whitten,  Pomology. 
*Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 
R.  L.  Adams,  Farm  Management. 

F.  L.  Griffin,  Agricultural  Education. 
John  E.  Dougherty,  Poultry  Husbandry. 
W.  B.  Herms,  Entomology  and  Parasitology. 

D.  R.  Hoagland,  Plant  Nutrition. 

L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 

DIVISION  OF   VITICULTURE  AND  FRUIT   PRODUCTS 

F.  T.  Bioletti  G.  Barovetto 

W.  V.  Cruess  A.  J.  Winkler 

A.  W.  Christie  J.  H.  Irish 
L.  O.  Bonnet 


*  In  cooperation  with  office  of  Public  Roads  and  Rural  Engineering,  U.  S.  Department  of 
Agriculture. 


DEHYDRATION  OF  FRUITS 

(A  PROGRESS  REPORT) 

By  W.  V.  C'KUESS  and  A.  W.  CHRISTIE. 


CONTENTS 

PAGE 

Introduction 50 

Definitions:  Drier,  Evaporator,  Dehydrater 52 

Preparation 52 

Ripening  of  Bartlett  Pears 53 

Effect  of  Maturity  on  Yield  and  Quality 53 

Comparison  of  Lye  Dipping  and  Blanching 54 

Pitting 55 

Peeling 55 

Slicing 56 

Tray  Capacity 56 

Sulfuring  57 

Yields  of  Dehydrated  Fruits 58 

Temperature 58 

Relative  Humidity 61 

Drying  Time 62 

Comparative  Yields  and  Qualities  of  Sun-Dried  and  Dehydrated  Fruits 63 

Comparison  of  Sun-Drying  and  Stack-Drying 65 

Sulf urous  Acid  Content  of  Sun-Dried  and  Dehydrated  Fruits 67 

Stemming 68 

Processing 69 

Moisture  Content 70 

Cost  of  Dehydration 71 

Miscellaneous  Fruits 72 

Figs 72 

Strawberries 73 

Loganberries 73 

Raspberries 74 

Olives 74 

Persimmons 74 

Bananas 74 

Citrus  Fruits 76 

Summary 77 

Acknowledgements 77 


50  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  production  of  dried  fruits  in  California  has  increased  rapidly 
during  the  past  decade.  The  total  rose  from  approximately  185,000 
tons  in  1910  to  approximately  360,000  tons  in  1920.  The  orchard  and 
vineyard  acreage  has,  moreover,  increased  notably  during  the  past  five 
years.  In  1919  and  1920,  about  275,000  acres  were  planted.  During 
the  1920  planting  season,  nurseries  were  unable  to  supply  the  demand 
for  trees  and  vines.  An  increased  output  of  fruit  is  therefore  to  be 
expected  within  the  next  three  to  five  years,  as  the  newly  planted 
orchards  and  vineyards  come  into  bearing. 

It  is  doubtful  whether  the  fresh  fruit  markets  and  canneries  will 
absorb  all  of  this  increase  and  even  under  the  most  favorable  market 
conditions,  it  is  probable  that  a  greater  proportion  than  at  present, 
of  certain  fruit  crops  must  be  preserved  and  marketed  in  the  dried 
form.  It  will  therefore  be  necessary  to  increase  our  markets  for  dried 
fruits  and  other  fruit  products  if  fruit  growing  is  to  continue  to  be 
profitable.  This  can  be  aided  by  improving  the  quality  of  these 
products,  particularly  of  the  dried  fruits,  and  by  diversifying  them. 
Dehydration  offers  a  means  of  producing  dried  fruits  of  new  forms 
and,  in  some  instances,  of  better  quality. 

The  sale  of  one  important  fruit  product  directly  affects  the 
marketing  of  other  fruit  products  and  of  fresh  fruit.  If  the  quality 
of  dried  fruit  is  high,  it  is  naturally  more  in  demand  and  commands 
a  higher  price  than  an  inferior  product.  However,  much  fruit  that 
is  unsuitable  for  marketing,  either  fresh  or  for  canning,  can  be  made 
into  an  acceptable  dehydrated  product  which  can  be  profitably  mar- 
keted. The  utilization  of  such  fruit  for  dehydration  prevents  waste 
and  aids  in  stablizing  the  market  for  all  fruit  products. 

In  California  all  fruits  except  apples  are  usually  dried  in  the  sun. 
Twenty-five  to  thirty  years  ago,  in  the  early  days  of  the  state's  fruit 
industry,  most  fruit  was  dried  by  artificial  heat.  It  has  been  stated 
that  wire  screen  trays  were  used  for  the  first  fruit  dried  in  the  sun 
in  order  that  the  product  should  bear  the  imprint  of  the  screen  in 
imitation  of  the  artificially  dried  product.  Some  sun-dried  fruit, 
however,  was  marketed  on  its  own  merits,  and  being  superior  to  the 
average  artificially  dried  fruit,  it  soon  came  into  greater  favor.  In 
time,  in  California,  sun-dried  prunes  and  raisins  displaced  the  product 
of  the  artificial  driers. 

Methods  of  dehydration  have  been  improved  recently  and  it  is 
now  contended  by  operators  of  modern  dehydraters  that  properly 
dehydrated  fruits  are  equal  or  even  superior  to  the  sun-dried  fruits, 
and  that  dehydration  possesses  certain  advantages  over  sun-drying. 


Bulletin  330  DEHYDRATION  OF  FRUITS  51 

The  advantages  claimed  are : 

1.  That  dehydrated  fruits,  when  cooked,  more  nearly  resemble  the 
fresh  frait  in  color  and  flavor. 

2.  That  dehydrated  fruits  are  produced  under  more  sanitary  con- 
ditions. 

3.  That  dehydration  permits  more  exact  control  of  quality  and 
yield. 

4.  That  less  land  and  fewer  trays  are  required  to  dehydrate  a 
given  tonnage  of  fruit. 

5.  That  dehydration  makes  it  possible  to  combine  all  the  steps  of 
drying  and  packing  in  one  building. 

In  seasons  of  early  rains  the  utility  of  dehydraters  in  preventing 
loss  of  fruit  and  in  reclaiming  rain-damaged  prunes  and  grapes  has 
been  fully  demonstrated.  It  has  also  been  conclusively  proved  that 
dehydraters  have  a  place  in  fruit-growing  sections  in  which  there  is 
even  in  normal  years  insufficient  sunshine  to  permit  successful  sun- 
drying. 

It  must  not  be  forgotten,  however,  that  California  has  established 
a  reputation  for  the  excellent  quality  of  her  sun-dried  fruits  and 
that  the  great  bulk  of  our  dried  fruits  will  probably  continue  to  be 
sun-dried,  however  successful  dehydration  may  become.  At  great 
expense  to  the  fruit-growing  associations,  a  widespread  demand  for 
"Sunsweet"  prunes  and  apricots,  "Blue  Ribbon"  sun-dried  peaches 
and  "Sun-Maid"  raisins  has  been  created.  It  is  believed  that  de- 
hydrated fruits  of  high  quality  will  find  new  markets  not  necessarily 
in  direct  competition  with  the  sun-dried  fruits  and  to  that  extent 
provide  an  increased  outlet  for  the  crops  of  our  rapidly  extending 
orchards. 

With  this  viewpoint  in  mind  and  also  to  test  the  validity  of  the 
claims  of  superiority  made  for  dehydration,  investigations  extending 
over  the  past  three  years  have  been  conducted  by  members  of  the 
Fruit  Products  Laboratory.  Though  the  investigations  are  not  yet 
complete,  the  widespread  interest  in  the  subject  makes  it  advisable 
to  publish  certain  important  results. 

This  publication  gives  the  results  of  investigations  and  observations 
on  the  dehydration  of  the  more  important  varieties  of  fruits  in  Cali- 
fornia. Pears,  peaches,  apricots,  and  grapes  were  dehydrated  experi- 
mentally in  commercial  quantities  at  the  University  Farm.  Other 
fruits  were  dried  there  in  lots  of  five  to  five  hundred  pounds  and  also 
in  the  laboratory  dehydraters  at  Berkeley.  Observations  and  experi- 
ments were  also  made  in  a  number  of  commercial  plants. 


52  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


DEFINITIONS 

Dehydration  is  at  present  denned  industrially  as  the  drying  of 
foods  by  artificially  produced  heat  under  carefully  controlled  con- 
ditions of  temperature,  humidity,  and  air  flow.  The  object  of  this 
process  may  be  accomplished  by  many  devices,  provided  they  conform 
to  certain  principles  governing  the  proper  drjdng  of  fruit.  The 
writers  published  in  1920  in  Bulletin  322  their  views  upon  the  term- 
inology of  dried  fruits.  At  that  time,  it  was  stated  that  the  term 
"dried"  should  apply  to  all  dried  fruits,  whether  sun-dried  or  dried 
by  artificial  heat,  and  that  "evaporated"  and  "dehydrated"  should 
be  of  equal  value  in  designating  fruits  dried  by  artificial  heat.  We 
see  no  reason  at  present  for  modifying  these  recommendations.  The 
fruit-drying  industry  itself,  however,  has  definitely  favored  the  word 
' '  dehydrated ' '  to  designate  artificially  dried  food  products  of  superior 
quality  in  preference  to  the  word  ' '  evaporated. ' '  It  therefore  appears 
that  commercial  usage  may  cause  the  adoption  of  the  former  term 
in  the  dried  fruit  trade. 

The  terms  drier,  dehydrater,  and  evaporator  are  now  used  more 
or  less  indiscriminately.  The  word  drier  is  usually  considered  to  be 
a  general  term  applicable  to  any  apparatus  used  to  remove  moisture 
from  fruit  or  other  materials.  A  dehydrater  is  usually  understood 
to  be  more  efficient  and  to  permit  of  more  careful  regulation  than  an 
evaporator. 

In  order  to  avoid  confusion,  the  following  definitions  are  used  in 
this  bulletin.  The  writers  do  not  consider  these  definitions  final  nor 
necessarily  authoritative. 

1.  Drier:  A  general  term,  applicable  to  all  machines  used  for 
drying  fruits  or  other  materials.  Examples :  hop  drier,  cement  drier, 
varnish  drier,  lumber  drier,  etc. 

2.  Evaporator:  A  drying  machine  without  forced  draft  and  which 
does  not  permit  of  accurate  control  of  temperature,  humidity,  or  air 
velocity. 

3.  Dehydrater:  A  drying  machine  with  forced  draft  and  in  which 
the  temperature,  relative  humidity,  and  air  velocity  can  be  accurately 
controlled. 

PREPARATION 

The  effect  of  various  methods  of  preparing  fruits  for  dehydration 
on  the  rate  of  drying  and  on  the  quality  of  the  finished  product  was 
studied  both  at  the  University  Farm  and  in  the  laboratory  at  Berkeley. 


Bulletin  330  DEHYDRATION   OF  FRUITS  53 

Ripening  of  Bartlett  Pears:  Approximately  3500  pounds  of  wind- 
fall and  cull  Bartlett  pears  from  a  single  orchard  were  divided  into 
three  portions.  One  lot  was  placed  in  lug  boxes  and  another  on 
screen  trays.  Both  lots  were  allowed  to  ripen  under  an  open  shed. 
The  third  lot  was  stratified  between  layers  of  straw  in  the  sun.  The 
fruit  in  each  lot  was  sorted  several  times  during  ripening. 

The  ripened  pears  were  sorted  for  removal  of  spoiled  fruit  and 
were  then  trimmed  and  halved  for  drying.  The  total  loss  from  sorting 
and  trimming  for  the  lot  ripened  on  screen  trays  was  7.4  per  cent; 
for  that  in  lug  boxes  8.6  per  cent,  and  for  the  lot  in  straw  8.5  per  cent. 

The  fruit  on  screen  trays  ripened  less  uniformly  than  that  in  lug 
boxes  or  straw  and  required  a  larger  number  of  sortings  during  ripen- 
ing. These  disadvantages  counterbalance  the  advantage  of  slightly 
smaller  loss  from  rot  in  this  method  of  ripening. 

Effect  of  Maturity  on  Yield  and  Quality:  Orchard  run  Muir 
peaches  were  sorted  into  three  lots  of  approximately  150  pounds  each. 
One  lot  represented  hard  green,  the  second  lot,  hard  ripe,  and  the 
third,  soft  ripe  fruit.  The  peaches  were  cut,  pitted,  spread  on  2'  X  3' 
field  trays,  and  sulfured  for  five  hours.  They  were  dried  in  the  sun 
for  three  days  and  then  in  the  stack  for  five.  Table  I  gives  the  yields 
of  dried  products  and  their  composition. 

TABLE  I 

Effect  of  Maturity  on  Yield  and  Composition  of  Dried  Peaches 


Per  cent 
of  pits 

Drying 
ratio 

Water 
per  cent 

in  dry 
product 

Lbs.  dry  fruit 
per  100  lbs.  fresh 

A 

Sugar  per  cent  in 
dried  fruit 

A 

Degree  of 
ripeness 

As 
weighed 

On  25 

per  cent 

water 

basis 

As 
weighed 

On  25 

per  cent 

moisture 

basis 

Soft  ripe 

6.5 

4.86:1 

18.1 

20.6 

22.5 

48.5 

43.9 

Hard  ripe 

5.9 

4.64:1 

19.4 

21.6 

23.2 

48.5 

45.1 

Hard  green 

4.3 

5.11:1 

18.1 

19.6 

21.5 

45.0 

41.2 

On  a  uniform  moisture  basis  of  25  per  cent  the  yield  from  green 
fruit  was  appreciably  less  than  that  from  ripe  fruit.  Likewise,  the 
sugar  content  of  the  former  was  considerably  less  than  that  of  the 
latter.  The  dried  green  fruit  was  astringent  and  sour  in  flavor  and 
of  an  unattractive  greyish  brown  color.  The  dried  hard-ripe  fruit 
was  lighter  in  color  and  of  less  pleasing  flavor  and  texture  than  the 
dried  soft-ripe  fruit.  The  quality  of  dried  pears  and  apricots  was 
affected  similarly  by  maturity. 

Muscat  and  Sultanina  (Thompson  seedless)  grapes  of  20°  Balling 
yielded   very   inferior   products    of   reddish    color    and   sour   flavor. 


54  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Grapes  of  the  same  varieties  at  25°  Balling  yielded  raisins  of  excellent 
flavor  and  appearance  by  similar  treatment. 

As  a  result  of  these  experiments  and  of  practical  experience,  it  is 
recommended  that  all  fruits  used  for  dehydration  be  thoroughly  ripe. 

Comparison  of  Lye  Dipping  and  Blanching:  Prunes  are  usually 
lye  dipped  before  dehydration,  although  in  some  plants  the  fruit  is 
dried  without  preliminary  treatment  and  in  others  the  prunes  are 
blanched  in  steam  before  drying.  In  order  to  compare  the  rate  of 
drying  of  untreated,  of  steamed,  and  of  lye-dipped  prunes,  an  experi- 
ment was  conducted  in  an  air-blast  laboratory  dehydrater.  One  lot 
was  dipped  in  boiling  1  per  cent  lye  solution,  another  steamed  on 
trays  in  a  heavy  jet  of  steam  for  five  minutes,  and  a  third  was 
untreated.  The  untreated  fruit  required  30  per  cent  more  time  to 
dry  than  the  dipped  prunes.  The  steamed  fruit  dried  slightly  more 
rapidly  than  the  dipped  fruit. 

The  tests  were  repeated  in  a  commercial  evaporator  in  which  the 
air  velocity  was  very  low.  There  was  little  difference  in  the  rates 
of  drying  in  this  instance,  although  the  untreated  fruit  dried  less 
evenly  than  that  which  had  been  steamed  or  dipped. 

In  a  large  natural  draft  evaporator,  dipped  prunes  lost  54  per  cent 
of  their  weight  in  31  hours  at  140°  to  145°  F.,  and  undipped  prunes 
under  the  same  conditions  lost  51  per  cent.  This  difference  is  small 
and  of  no  practical  significance. 

In  a  commercial  dehydrater  in  which  the  velocity  of  the  air  across 
the  trays  was  high  (about  600  feet  per  minute)  dipped  prunes  dried 
in  approximately  one-half  the  time  required  for  untreated  fruit. 

From  these  experiments,  it  seems  evident  that  lye  dipping  does  not 
appreciably  hasten  drying  when  the  air  velocity  is  low,  but  does  so 
materially  when  the  air  velocity  is  high.  Undipped  prunes  dry  prac- 
tically as  rapidly  as  dipped  prunes  in  evaporators  of  low  air  velocity 
because  the  limiting  factor  in  such  instances  is  probably  not  the 
ability  of  the  fruit  to  give  up  its  water  rapidly,  but  rather  the  limited 
capacity  of  the  low  air  flow  to  supply  sufficient  heat  for  rapid  evapora- 
tion of  the  moisture. 

Dehydrated  steamed  prunes  were  redder  in  color  and  more 
translucent  than  dehydrated  lye-dipped  prunes.  Prolonged  steaming 
caused  bursting  of  the  fruit  and  sticking  to  the  trays.  With  ample 
steam  supply,  two  to  five  minutes'  exposure  to  steam  was  found  suf- 
ficient, but  in  instances  where  the  steam  supply  was  inadequate,  a 
longer  period  of  treatment  was  required.  The  fruit  should  be  heated 
through  completely,  but  heating  should  not  be  continued  long  enough 
to  unduly  soften  it. 


Bulletin  330  DEHYDRATION  OF  FRUITS  55 

Certain  varieties  of  grapes,  such  as  Tokays  and  Emperors,  re- 
sponded satisfactorily  to  steaming.  Wine  grapes,  such  as  Zinfandel 
and  Alicante  Bouschet  varieties,  softened  badly  when  steamed  and 
lost  a  great  deal  of  their  juice.  For  a  dehydrater  in  which  all  common 
varieties  of  grapes  are  to  be  dried,  lye  dipping  is  more  satisfactory 
than  steaming.  Plants  in  which  only  Tokay  grapes  are  dried  can  use 
steaming  to  advantage. 

Dipping  for  5  to  20  seconds  in  a  boiling  2  per  cent  sodium  car- 
bonate solution,  followed  by  rinsing  in  water,  checked  the  skins  of 
Royal  Anne  and  Black  Tartarian  cherries  more  satisfactory  than  did 
dilute  lye  solutions.  The  lye  tended  to  peel  the  more  tender  fruit, 
although  good  results  were  obtained  commercially  by  dipping  cull 
cherries  for  10  to  30  seconds  in  a  boiling  solution  containing  1  per  cent 
of  Canner's  alkali,  a  mixture  of  lye  and  carbonate.  Dipping  reduced 
the  drying  time  approximately  one  half  and  was  found  to  be  more 
satisfactory  than  blanching  in  steam  or  hot  water. 

Pitting:  All  peaches  and  apricots  dehydrated  at  the  University 
Farm  in  1920  were  pitted  by  hand  before  drying.  Royal  apricots 
yielded  7.7  per  cent  of  fresh  pits  and  5.9  per  cent  of  sun-dried  pits 
upon  the  basis  of  the  fresh  uncut  fruit. 

Muir  peaches  yielded  6.1  per  cent  of  fresh  pits  and  5.3  per  cent 
of  sun-dried  pits. 

Small  quantities  of  Royal  Anne  and  Black  Tartarian  cherries  were 
pitted  with  a  hand  power  pitter,  causing  a  loss  in  each  instance  of 
18  per  cent  of  the  fresh  weight. 

Peeling:  Halved  Muir  peaches  were  peeled  by  immersion  for  30 
seconds  in  boiling  5  per  cent  lye  solution  and  rinsing  in  cold  water 
to  remove  adhering  lye  and  softened  skins.  -The  loss  in  lye  peeling 
was  4.5  per  cent  of  the  weight  of  the  pitted  fruit.  The  lye-peeled 
fruit  required  less  sulfuring,  dried  more  rapidly,  and  yielded  a  more 
attractive  finished  product  than  did  the  unpeeled  fruit.  Exposure 
to  sulfur  fumes  for  one  hour  was  sufficient.  The  lye-peeled  fruit 
darkened  rapidly  unless  placed  in  the  sulfur  fumes  very  soon  after 
peeling.  Dipping  in  dilute  salt  solution  after  peeling,  reduced  the 
tendency  to  darken.  Lye  peeled  peaches  gave  trouble  by  sticking 
to  wooden  trays.  This  difficulty  can  be  overcome  by  covering  the 
to  wooden  trays.  The  application  of  "slab"  oil  to  the  trays  did  not 
prevent  sticking. 

Whole  ripe  Bartlett  pears  required  an  immersion  of  20  seconds 
in  boiling  10  per  cent  lye  solution  and  firm  ripe  pears  30  to  40  seconds. 
Green  fruit  did  not  respond  satisfactorily  to  lye  peeling.  After  dip- 
ping in  the  boiling  lye  solution,  the  skins  were  removed  by  agitation 


56  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

of  the  fruit  in  a  wire-screen  basket  in  cold  water.  The  peeled  fruit 
was  then  halved  and  cored.  The  loss  by  lye  peeling  was  20  per  cent ; 
by  lye  peeling  and  coring,  34  per  cent,  and  by  hand  peeling  and 
coring  36  per  cent.  It  was  found  necessary  to  immerse  the  lye-peeled 
fruit  in  dilute  brine  (3  to  5  per  cent  salt)  or  to  wet  the  fruit  on  the 
trays  with  this  solution,  in  order  to  prevent  darkening  before  it  was 
placed  in  the  sulfur  house.  Peeled  pears  required  only  one  hour  of 
sulfuring  as  compared  with  24  hours  for  the  unpeeled  fruit.  The 
peeled  and  cored  pears  yielded  a  very  attractive  dehydrated  product 
which,  after  cooking,  closely  resembled  canned  pears. 

A  narrow  strip  of  peel  cut  around  the  pear  from  stem  to  calyx, 
followed  by  halving  and  coring,  prevented  the  edges  of  the  fruit  from 
curling  during  drying.  This  product  was  much  superior  in  appear- 
ance to  fruit  that  was  merely  halved  without  removal  of  stem,  core, 
or  the  narrow  strip  of  peel. 

Slicing:  All  fruits  dry  more  rapidly  when  cut  in  thin  pieces.  Less 
sulfuring  is  required  and  the  finished  product  cooks  more  rapidly 
than  the  whole  or  halved  fruit.  Sliced  dehydrated  pears,  apricots, 
persimmons,  and  peaches  were  very  attractive  in  appearance  and 
decidedly  "different"  from  the  usual  sun-dried  fruits.  Thinly  sliced 
or  cubed  dehydrated  apples  were  superior  for  culinary  purposes  to 
the  usual  ring  style. 

Sliced  Bartlett  pears  were  dehydrated  in  less  than  6  hours,  lye- 
peeled  halves  in  16  to  18  hours,  and  unpeeled  halves  in  36  to  48  hours 
under  similar  drying  conditions.  The  sliced  product  was  excellent 
when  used  for  sauces  or  pies. 

As  a  result  of  these  experiments,  the  writers  wish  to  call  attention 
to  the  excellence  of  apricots,  peaches,  pears,  and  apples  that  have 
been  thinly  sliced,  cubed,  or  shredded  before  dehydration. 

Tray  Capacity:  The  amount  of  fruit  that  may  be  placed  upon  each 
square  foot  of  tray  surface  varies  greatly  with  the  variety  and  size 
of  the  fruit,  its  method  of  preparation,  and  the  system  of  dehydration 
employed. 

At  the  University  Farm,  the  dehydrater  trays  held  two  pounds 
of  medium-size  halved  apricots  per  square  foot;  three  pounds  of 
medium-size  halved  Muir  peaches,  three  pounds  of  halved  pears,  two 
and  one-half  to  three  and  one-half  pounds  of  prunes  one  layer  deep, 
and  three  to  four  pounds  of  grapes.  Thinly  sliced  fruit  tends  to  form 
masses  on  the  tray  which  impede  air  flow  and  thereby  retard  drying. 
Consequently,  trays  should  be  loaded  less  heavily  with  such  fruit  than 
with  large  pieces. 


Bulletin  330  DEHYDRATION  OF  FRUITS  57 

In  dehydraters  using  high  air  velocity  (500-1000  feet  per  minute) 
and  high  initial  temperatures  (190°  F.,  or  above)  the  trays  can  be 
much  more  heavily  loaded  than  where  lower  air  velocities  and  lower 
temperatures  (110°  to  170°  F.)  are  used.  For  example,  trays  of 
dipped  cherries  containing  two  and  four  pounds  per  square  foot 
respectively,  dried  equally  rapidly  in  a  laboratory  dehydrator,  using 
an  initial  temperature  of  210°  F.,  a  finishing  temperature  of  170°  F., 
and  an  air  velocity  of  1000  feet  per  minute.  Similar  results  were 
obtained  with  sliced  apples. 

At  the  temperatures  and  air  velocities  at  present  employed  in 
commercial  dehydration,  the  rate  of  drying  is  very  materially  affected 
by  the  load  per  square  foot  of  tray  surface.  The  loads  per  square 
foot  of  tray  surface  recommended  for  various  fruits  to  be  dehydrated 
by  the  counter  current  system  are  given  in  Table  VIII. 

Sulfuring:  The  natural  color  of  most  fruits  is  retained  by  exposure 
of  the  prepared  fruit  to  the  fumes  of  burning  sulfur  before  drying. 
The  time  of  sulfuring  necessary  varies  with  the  variety  of  fruit  and 
its  previous  treatment.  For  example,  thinly  sliced  Bartlett  pears 
require  20  to  30  minutes  sulfuring,  tye-peeled  pears  1  to  3  hours, 
and  unpeeled  halved  pears  24  to  36  hours.  Sliced  apricots  require 
30  minutes  sulfuring  as  compared  to  one  hour  for  the  halved  fruit. 
Immersion  of  the  cut  fruit  in  dilate  brine  reduces  the  time  of  sulfur- 
ing required,  especially  for  white  fruits,  such  as  pears  and  apples. 

As  a  result  of  the  investigations  of  the  past  two  seasons,  the  periods 
of  sulfuring  given  in  Table  VIII  are  recommended  for  the  various 
fruits. 

Galvanized  screen  trays  have  proved  unsatisfactory  for  the  drying 
of  fruits  that  require  sulfuring.  The  sulfurous  acid  formed  by  solu- 
tion of  sulfur  dioxide  (sulfur  fumes)  in  the  fruit  juices  rapidly  dis- 
solves the  zinc-  coating,  imparting  a  metallic  flavor  to  the  fruit  and 
exposing  the  iron  wire  of  the  screen.  Iron  causes  blackening  of  white 
fruits  by  the  reaction  of  iron  salts  with  the  fruit  tannins.  Screen  trays 
are  shortlived  when  used  for  sulfured  fruits,  and  must  be  replaced  fre- 
quently. Attempts  have  been  made  to  coat  the  trays  with  paraffin, 
slab  oil,  varnish,  can  lacquer,  various  paints,  and  other  protective 
coatings,  but,  to  date,  no  satisfactory  material  has  been  found. 
Wooden  slat  trays  have  given  excellent  service  in  the  University  Farm 
and  other  dehydraters.  They  are  not  corroded  and  are  less  expensive 
than  screen  trays. 

There  is  a  growing  demand  for  unsulfured  dried  fruit.  Untreated, 
sun-dried,  or  dehydrated  cut  fruits,  such  as  apples,  peaches,  etc.,  are 
of  an  unattractive  brown  color.     Sliced  fruits  retain  much  of  their 


58  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

original  color  without  excessive  browning  if  immersed  in  cold  dilute 
(3  to  5  per  cent)  salt  solution  for  several  minutes  immediately  after 
cutting.  Unpeeled  halved  fruit  retains  its  fresh  color  on  the  cut 
surface  but  darkens  beneath  the  skin.  Blanching  on  the  trays  in 
steam  gives  fairly  satisfactory  results  with  white  grapes,  Tokay  grapes, 
and  with  peeled  halved  pears  and  peaches,  but  fails  to  produce  an 
attractive  color  in  unpeeled  halved  pears,  peaches,  and  apricots. 
Steaming  softens  berries  and  apples  so  much  that  the  dried  product 
is  of  very  unattractive  appearance.  Berries,  cherries,  prunes,  and 
persimmons  retain  their  color  very  well  without  sulfuring  before 
drying. 

YIELDS    OF    DEHYDRATED    FRUITS 

Yields  of  dehydrated  grapes,  peaches,  pears,  and  apricots  were 
determined  at  the  University  Farm  on  lots  of  five  to  thirty-five  tons 
of  fresh  fruit.  Yields  for  a  number  of  other  fruits  were  determined 
in  the  laboratory.    These  data  are  given  in  Table  II. 

The  yields  vary  according  to  the  locality  in  which  the  fruit  is 
grown,  the  season,  the  maturity  of  the  fruit,  and  the  variety.  For 
example,  the  average  drying  ratio  for  all  grape  varieties  dried  at  the 
University  Farm  in  1919  was  approximately  3  :1,  and  in  1920,  it  was 
3.64:1.  It  must  be  expected,  therefore,  that  considerable  variation 
from  the  yields  given  in  Table  II  will  be  found  in  practice.  The 
data  are  useful,  however,  in  indicating  the  approximate  comparative 
yields  of  various  fruits  and  the  loss  in  preparation. 

TEMPERATURE 

The  temperature  of  the  air  used  in  dehydration  not  only  greatly 
affects  the  time  required  for  drying,  but  also  the  quality  of  the  finished 
product.  In  order  to  secure  large  capacity  and  minimum  operating 
costs,  it  is  necessary  to  use  the  highest  temperature  that  will  not 
materially  injure  the  product.  Practically  all  dehydraters  which  in- 
volve a  progressive  movement  of  the  fruit  through  the  drying  chamber 
have  used  the  "counter  current  system,"  which  means  that  the  fruit 
is  introduced  into  relatively  cold  moist  air  (100°  F.  to  130°  F.)  and 
moved  toward  a  region  of  warmer,  drier  air  until  the  drying  is  com- 
pleted at  temperatures  of  150°  F.  to  180°  F.  Recent  tests  on  a 
commercial  scale  indicate  that  this  system,  at  least  for  certain  fruits, 
is  not  so  efficient  as  the  "parallel  current  system"  discussed  in 
another  report  of  this  Station.  The  "critical  temperature"  for  any 
fruit  is  the  temperature  at  which,  when  the  fruit  is  almost  dry,  it  may 
undergo  undesirable  changes  in  color  or  flavor.     In  the   "counter 


Bulletin  330  DEHYDRATION   OF  FRUITS  59 

TABLE  II 

Shrinkage  in  the  Preparation  and  Dehydration  of  Various  Fruits 

Per  cent  of  fresh  fruit  unsorted  Drying  ratio 

, A ,  , * , 

Prepared 

fruit  cut,     Prepared  Gross  Prepared 

Sorted  pitted,  and  Loss  in  fresh  fresh 

Fruit  uncut  dipped  or       peeled         prepara-         Dried  to  to 

fruit  hulled  fruit  tion  fruit  net  dry  net  dry 

Eoyal  Apricots 

from  Winters      100  92.3  7.7  17.2  5.8:1  5.4:1 

Muir  peaches 

from  Winters, 

unpeeled  96.4  90.3  9.7  20.9  4.8:1,         4.3:1 

Muir  peaches 

from  Winters, 

lye  peeled  96.4  90.3  86.2  13.8  19.9  5.0:1  4.3:1 

Bartlett  pears 

from  Sacramento, 

unpeeled  97.9  91.7  8.3  19.5  5.1:1  4.7:1 

Bartlett  pears 

from  Sacramento, 

lye  peeled 

and  cored  97.9  91.7  60.5  39.5  12.9  7.8:1  4.7:1 

Grapes,  all 

varieties, 

stemmed, 

Univ.Farm, 

1920  100  100  100  0  27.5  3.6:1  

Newtown 

apples  from 

Watsonville  100  75  25  12.3  8.3:1  6.1:1 

Loganberries 

from 

Sebastopol  100  100  100  0  21.1  4.7:1  4.7:1 

Cherries,  Black 

Tartarian  from 

San  Leandro, 

not  pitted  100  100  100  0  33.5  3.0:1  3.0:1 

Cherries,  Black 

Tartarian,  from 

San  Leandro, 

pitted  and 

stemmed  100  80  20  23.7  4.2:1  3.4:1 

Cherries, 

Eoyal  Anne,  » 

from  Napa, 

not  pitted  100  27.7  3.6:1  

Cherries, 

Eoyal  Anne, 

from  Napa, 

pitted  and 

stemmed  100  80  20  23.5  4.3:1  3.4:1 

Easpberries 

from 

Oakland  100  100  100  0  14.8  6.8:1  6.8:1 

Strawberries, 

four  varieties, 

from  Salinas       100  96.4  3.6  16.7  5.9:1  


60  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

current  system,"  this  temperature  is  the  maximum  which  can  be 
used,  while  in  the  "parallel  current  system,"  this  temperature  must 
not  be  exceeded  in  the  final  stages  of  drying,  although  much  higher 
temperatures  can  be  used  while  the  fruit  still  contains  an  excess  of 
moisture. 

The  maximum  advisable  finishing  temperatures  for  each  of  the 
more  important  fruits  are  given  in  Table  VIII.  These  temperatures 
were  obtained  during  the  operation  of  the  University  Farm  and  other 
dehydraters  operated  on  the  "counter  current  system."  It  is  likely 
that  these  temperatures  may  be  seriously  modified  during  the  1921 
season  by  the  use  of  the  "parallel  current  system."  In  the  latter 
method,  the  critical  or  finishing  temperature  is  accompanied  by  a  rela- 
tively high  humidity  which  partially  protects  the  fruit  from  injury, 
whereas  in  the  "counter  current  system"  the  air  at  the  finishing 
temperature  is  generally  rather  dry,  a  condition  more  conducive  to 
injury. 

Experiments  by  Gadgil,  Winkler,  and  Bjarnason,  graduate 
students  in  the  Fruit  Products  laboratory,  indicated  rapid  loss  of 
sugar  when  raisins  were  heated  to  185°  F.  after  becoming  nearly  dry. 
At  lower  temperatures,  the  effects  were  negligible  unless  the  raisins 
were  allowed  to  become  very  much  over-dried,  a  condition  which 
should  never  occur  in  a  commercial  plant.  The  extent  of  such  sugar 
loss  is  indicated  in  Table  III.  Another  test  in  a  commercial  plant 
showed  that  Alicante  Bouschet  grapes  dried  at  190°  F.  to  200°  F., 
to  10  per  cent  moisture,  for  stemming,  contained  5  per  cent  less  sugar 
than  grapes  from  the  same  lot  dried  to  the  same  moisture  content  at 
165°  F. 

Apricots  have  been  finished  at  175°  F.,  but  this  temperature  is  apt 
to  cause  injury.  For  best  results,  165°  F.  should  not  be  exceeded. 
Peaches  are  a  little  more  sensitive  to  injury  than  apricots,  especially 
in  the  pit  cavity  and  on  the  hairy  skin.  For  this  fruit,  160°  F.  is 
considered  the  highest  temperature  advisable  during  the  last  stages 
of  deh}'dra£ion.  Pears  finished  above  145°  F.  become  yellowish  brown 
when  nearly  dry.  Therefore,  if  a  white  product  is  desired,  145°  F. 
should  not  be  exceeded.  If  a  translucent  product  similar  to  the  highly 
sulfured  sun-dried  pear  is  desired,  drying  should  be  conducted  at 
110°  F.  to  120°  F.,  although  such  an  operation  would  probably  not 
be  economical.  Prunes  can  not  be  safety  finished  above  170°  F., 
although  temperatures  as  high  as  190°  F.  have  been  successfully  used 
where  the  prunes  dried  very  evenly  and  were  removed  from  the 
dehydrater  while  still  containing  over  20  per  cent  moisture.  The 
grapes  dehydrated  at  the  University  Farm  were  finished  at  165°  F., 


Bulletin    330  DEHYDRATION    OF   FRUITS  61 

which  gave  an  exceptionally  good  product  with  natural  color  and 
flavor.  Wine  grapes  dehydrated  commercially  at  185°  F.  had  brown- 
colored  flesh  and  a  carmelized  flavor.  Sliced  sulfured  apples  will 
withstand  160°  P.  when  dry,  without  browning,  while  at  180°  F.  the 
product  turns  brown  even  before  sufficiently  dry.  Fresh  apples  were 
not  injured  at  220°  F.,  while  still  high  in  moisture.  Both  black  and 
white  cherries  were  successfully  finished  at  170°  F.,  using  either  the 
parallel  or  counter  current  systems. 

TABLE  III 

LOSS    OF    SUGAR   FROM    RAISINS    SUBJECTED    TO    VARIOUS    TEMPERATURES 


Temperature 

Hours  exposed 

Per  cent  of 
sugar  loss 

140°  F. 

8 

0.6 

140°  F. 

16 

0.8 

140°  F. 

32 

1.0 

167°  F. 

8 

1.3 

167°  F. 

16 

1.9 

167°  F. 

32 

6.2 

185°  F. 

8 

8.7 

185°  F. 

16 

12.2 

185°  F. 

32 

14.9 

RELATIVE    HUMIDITY 

The  rate  of  evaporation  from  a  free  water  surface  in  air  at  a 
given  temperature  varies  inversely  with  the  relative  humidity  of  the 
air;  that  is,  the  higher  the  relative  humidity,  the  less  rapid  the  rate 
of  evaporation.  Relative  humidity  of  air  may  be  defined  as  its  per- 
centage of  saturation  with  moisture  vapor.  Air  completely  saturated 
with  water  vapor  at  a  given  temperature  is  at  100  per  cent  relative 
humidity ;  air  at  the  same  temperature  containing  one-half  the  amount 
of  water  vapor  that  it  is  capable  of  absorbing  is  at  50  per  cent  relative 
humidity.  The  absolute  amount  of  water  vapor  that  air  can  absorb 
(within  certain  temperature  limits)  approximately  doubles  with  each 
27°  F.  rise  in  temperature.  For  example,  air  at  177°  F.  and  25  per 
cent  relative  humidity  contains  twice  as  much  water  vapor  as  air  at 
150°  F.  and  25  per  cent  relative  humidity.  In  other  words,  the  higher 
the  temperature,  the  greater  is  the  moisture  carrying  capacity  of  the 
air.  Therefore,  theoretically,  high  temperatures  and  low  relative 
humidities  should  be  employed  in  dehydration. 

The  evaporation  of  water  from  many  fruits,  however,  does  not 
follow  closely  the  law  of  evaporation  of  water  from  a  free  surface. 
Large  pieces  of  fruit,  such  as  halved  pears,  peaches,  or  whole  Imperial 


62  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

prunes,  case-harden  if  the  relative  humidity  is  so  low  and  the  tem- 
perature so  high  that  the  moisture  is  removed  more  rapidly  from  the 
surface  than  it  diffuses  from  the  interior  of  the  fruit.  Case-hardening 
retards  the  rate  of  evaporation  because  it  impedes  the  diffusion  of 
water  to  the  surface  of  the  fruit. 

In  experiments  on  pears  and  peaches,  it  was  found  that  case- 
hardening  was  materially  reduced  by  increasing  the  relative  humidity 
of  the  air  at  150°  F.  to  30  to  35  per  cent.  A  humidity  of  20  to  25 
per  cent  was  obtained  by  recirculation  of  the  exhaust  air  and  the 
remaining  10  to  15  per  cent  was  gained  by  the  introduction  of  fine 
sprays  of  water  into  the  heating  chamber. 

Thinly  sliced  fruits  (e.g.  pears,  apricots,  and  peaches),  dipped 
grapes,  small  to  medium  size  prunes,  and  dipped  cherries  do  not  case- 
harden  seriously.  Therefore,  air  of  high  initial  temperature  (190° 
to  210°  F.)  and  low  relative  humidity  can  be  employed  successfully 
with  such  fruits.  The  minimum  relative  humidities  of  air  at  the  finish- 
ing temperature  found  satisfactory  for  the  various  fruits  are  given 
in  Table  VIII,  Column  5. 

DRYING   TIME 

The  time  required  for  the  dehydration  of  a  given  fruit  varies 
considerably  with  the  variety,  degree  of  maturity,  preliminary  treat- 
ment, temperature,  humidity  and  volume  of  air,  tray-load,  degree  of 
dryness  desired,  and  other  factors.  It  is  therefore  impossible  to  give 
a  standard  drying  time  for  each  fruit.  The  following  times,  however, 
observed  in  the  University  Farm  dehydrater  and  other  successful 
commercial  dehydraters  may  be  considered  as  normal.  Unless  other- 
wise noted,  these  times  refer  only  to  the  air-blast  tunnel  dehydrater 
operated  on  the  counter  current  system. 

Using  a  finishing  temperature  of  165°  F.,  apricots  were  dehydrated 
in  from  9  to  15  hours,  averaging  12  hours.  At  160°  F.,  unpeeled 
peaches  required  20  to  30  hours,  averaging  24  hours,  while  peeled 
peaches  required  only  14  hours.  Unpeeled  pears  dried  in  36  to  48 
hours  when  finished  at  150°  F.  Sliced  pears  required  only  6  to  8 
hours  and  peeled  halves  16  to  18  hours  at  the  same  temperature. 
Drying  time  for  French  prunes  varied  from  18  to  30  hours,  except  in 
natural  draft  evaporators  in  which  30  to  36  hours  was  normal.  The 
average  drying  time  in  air-blast  dehydraters  in  1920  was  24  hours 
for  prunes,  based  on  ample  air  flow  and  a  finishing  temperature  of 
165°  F.  At  least  6  hours  should  be  added  to  these  times  in  the  case 
of  Imperial  prunes.     The  maximum  drying  time  for  any  variety  of 


Bulletin   330  DEHYDRATION  OF  FRUITS  63 

grapes  at  the  University  Farm  was  30  hours.  These  grapes  had  been 
lye-dipped  and  were  finished  at  165°  F.  Most  wine  grapes  required 
only  24  hours  and  Sultanina  (Thompson  seedless)  grapes  were  dried 
in  15  hours.  Sliced  apples  did  not  require  over  8  hours  to  dry  in  an 
air-blast  dehydrater  at  160°  F.,  while  in  a  stack  evaporator  18  hours 
were  needed.  By  using  a  high  initial  temperature  (200°  F.)  the  time 
can  be  reduced  to  2  hours.  Using  the  parallel  current  system,  starting 
at  210°  F.  and  finishing  at  170°  F.,  dipped  cherries  were  dried  in  4 
to  5  hours. 


COMPARATIVE    YIELDS    AND    QUALITIES    OF    SUN-DRIED    AND 
DEHYDRATED    FRUITS 

Accurately  controlled  comparisons  of  sun-drying  and  dehydration 
of  apricots,  peaches,  pears,  and  prunes  were  made  at  the  University 
Farm.  Uniform  lots  of  several  hundred  pounds  of  each  fruit  were 
used.  With  the  cut  fruits,  the  two  halves  of  each  individual  fruit 
were  placed  on  separate  trays,  one  lot  of  several  trays  being  sun- 
dried  in  the  customary  way  and  the  other  lot  dehydrated  during 
regular  operation  of  the  dehydrater.  Before  drying,  both  lots  were 
sulfured  the  usual  time  for  sun-dried  fruit.  Prunes  were  uniformly 
mixed  and,  after  dipping  and  rinsing,  were  divided  equally  between 
field  and  dehydrater  trays.  By  such  precautions,  fruit  of  the  same 
condition  was  used  for  the  two  methods  of  drying  and  any  subsequent 
differences  in  the  dried  product  were  a  direct  result  of  the  method  of 
drying. 

After  drying  the  fruit  was  again  carefully  weighed  and  represent- 
ative samples  withdrawn  which  were  later  analyzed  for  moisture  and 
sugar.    The  figures  obtained  are  given  in  Table  IV. 

In  the  case  of  peaches  and  apricots,  dehydration  gave  a  slightly 
greater  weight  of  dried  fruit,  but  when  considered  on  a  uniform 
moisture  content  of  25  per  cent,  this  greater  yield  was  shown  to  be 
entirely  due  to  the  higher  moisture  content  allowed  to  remain  in  the 
dehydrated  product.  In  fact,  the  sun-dried  lots  show  slightly  greater 
yields  of  dry  matter  which  is  at  least  partly  to  be  attributed  to  dust 
and  sand  accumulated  during  drying.  With  prunes,  the  sugar  deter- 
minations reveal  no  significant  differences  between  the  two  methods 
of  drying.  Various  lots  were  dried  so  nearly  to  the  same  moisture 
content  that  the  slight  differences  in  yield  noted  are  considered  to  be 
within  the  experimental  error.  No  moisture  or  sugar  determinations 
were  made  on  the  dried  pears,  but  it  was  obvious  that  the  lower  yield 


64  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

of  dehydrated  pears  was  due  principally  to  their  lower  moisture  con- 
tent, caused  by  over-drying.  Similar  results  were  obtained  on  grapes 
in  1919. 

The  dehydrated  apricots  were  graded  as  "Growers  Brand"  by  the 
California  Prune  and  Apricot  Growers,  Inc.,  and  brought  IV2  cents 
per  pound  less  than  "Sunsweet  Brand."  This  discrimination  was 
made  because  the  dehydrated  fruit  retained  the  color  which  the  fruit 


TABLE  IV 

Comparative   Yields 

by   Sun-Drying 

and   Dehydration   of 

Various 

Fruits 

Pounds  dry  fruit  per 

Per  cent  of  sugar 

100  pounds  fresh 

in  dry  fruit 

IV^oistiirG 

A 

A 

in  dry 

As 

On  25% 

As 

On  25% 

Fruits  and 

Drying 

fruit, 

weighed 

water 

weighed 

water 

method  of  drying 

ratio 

per  cent 

from  tray 

basis 

from  tray 

basis 

Apricots 

In  sun 

5.1:1 

15.6 

19.7 

22.1 

50.1 

47.5 

In  Univ.  dehyclrater 

4.6:1 

25.8 

21.6 

21.4 

43.8 

47.2 

Apricots 

In  sun 

5.3:1 

14.5 

18,9 

21.6 

48.3 

45.2 

In  stack  evaporator 

4.7:1 

28.4 

21.0 

20.0 

40.2 

44.9 

Peaches 

In  sun 

4.8:1 

17.8 

20.9 

22.9 

49.1 

44.8 

In  Univ.  dehyclrater 

4.5:1 

23.7 

22.1 

22.5 

47.9 

47.0 

Prunes 

In  sun 

1.5:1 

15.7 

68.0 

76.2 

In  Univ.  dehydrater 

1.5:1 

14.9 

66.7 

75.7 

Prunes 

In  sun 

1.5:1 

15.0 

66.2 

74.8 

In  Univ.  dehydrater 

1.5:1 

14.4 

67.1 

76.3 

Pears 

In  sun 

4.6:1 

21.5 

In  Univ.  dehydrater 

5.0:1 

20.1 

Peaches 

In  sun 

4.5:1 

17.8 

22.5 

24.6 

49.1 

44.9 

In  stack 

4.5:1 

17.3 

22.5 

24.8 

48.3 

43.9 

had  at  the  time  of  cutting.  Green  colored  fresh  fruit  came  from  the 
dehydrater  green  in  color,  while  similar  fruit  placed  in  the  sun 
acquired  a  uniform  golden  yellow  color,  although  still  ''green'"  in 
flavor.  The  deh37drated  fruit  was  opaque  rather  than  translucent. 
When  green  apricots  were  avoided  in  picking,  the  thoroughly  ripe 
fruit  yielded  a  dehydrated  product  of  a  beautiful  golden  color,  rival- 
ing if  not  surpassing  the  sun-dried  fruit.  In  quality,  the  dehydrated 
fruit  was  found  much  superior  to  the  sun-dried.  It  absorbed  water 
quickly  and  regained  its  full  size  during  overnight  soaking  in  water. 
It  cooked  more  quickly  than  the  sun-dried  article  and  when  used  in 
sauces,  pies,  or  puddings,  very  closely  resembled  fresh  apricots  pre- 
pared in  the  same  way. 


Bulletin  330  DEHYDRATION  OF  FRUITS  65 

Profiting  by  the  previous  experience  with  apricots,  only  thoroughly 
ripe  peaches  were  dehydrated,  the  product  being  accepted  as  first 
grade  at  the  packing-house.  The  packer  stated,  however,  that  the 
dehydrated  peaches  were  inferior  in  color  and  less  "springy"  than 
the  sun-dried,  which  merely  meant  that  the  product  was  different  from 
the  customary  sun-dried  peaches.  After  refreshing  in  water,  and 
cooking,  the  dehydrated  peaches  possessed  a  color  and  a  flavor  more 
closely  resembling  those  of  the  fresh  fruit  than  did  the  sun-dried. 
The  peeled  dehydrated  peaches  were  especially  good. 

Dehydrated  prunes  are  in  general  lighter  in  color  than  the  sun- 
dried.  This  is  particularly  true  of  the  flesh,  which  is  invariably  of  a 
light  amber  color.  Dehydrated  prunes  are  usually  cleaner  and  have 
a  bright  glossy  appearance.  When  cooked,  the  flavor  is  excellent, 
resembling  more  closely  that  of  the  cooked  fresh  fruit.  This  may  or 
may  not  be  an  advantage,  depending  on  the  consumer's  preference  for 
the  usual  sun-dried  product. 

Dehydrated  unpeeled  pears,  even  when  sulfured  72  hours  before 
drying,  were  not  translucent  like  sun-dried  pears  but  were  chalky 
white.  If  the  fruit  is  first  partially  dried  in  the  sun  and  finished  in 
a  dehydrater,  a  translucent  product  is  obtainable.  Dehydrated  pears, 
especially  peeled  and  cored  halves,  are  greatly  superior  in  flavor  to 
sun-dried  pears,   closely  resembling  fresh  fruit  after  cooking. 

Dehydrated  wine  grapes  are  unquestionably  superior  to  the  sun- 
dried  for  the  making  of  beverages  since  they  yield  a  juice  of  fresh 
color  and  flavor  after  being  soaked  in  water  and  pressed.  On  the  other 
hand,  dehydrated  Muscat  grapes,  even  when  dried  without  prelim- 
inary dipping  or  sulfuring,  do  not  resemble  the  sun-dried  raisins  in 
flavor.  In  all  instances,  but  especially  when  the  grapes  were  dipped 
and  sulfured,  the  dehydrated  product  retained  a  lighter  color  and  a 
fresh  Muscat  flavor  and  not  the  well  known  raisin  flavor  resulting 
from  sun-drying.  The  dehydrated  Muscat  can  not  be  considered  a 
direct  competitor  of  "Sun-Maid"  raisins,  but  is  nevertheless  an 
excellent  product,  which  may  in  time  create  for  itself  a  distinct 
market.  A  similar  comparison  may  be  made  between  dehydrated  and 
sun-dried  Sultanina  (Thompson  Seedless)  grapes,  although  because  of 
the  lack  of  a  distinctive  flavor  in  these  grapes,  the  differences  were 
principally  in  appearance  rather  than  in  flavor.  Dehydrated  bleached 
Sultaninas  were  not  translucent  like  the  sun-dried,  but  otherwise  were 
an  excellent  product. 

Comparison  of  Sun-Drying  and  Stack-Drying :  A  comparison  of 
sun-drying  and  stack-drying,  using  peaches,  was  made  in  the  same 
way  as  between  sun-drying  and  dehydration.     One  lot  was  spread  in 


66 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


the  sun  for  three  days  and  the  trays  were  then  stacked  for  five  days, 
the  total  drying  time  being  eight  days.  The  second  lot  was  stacked 
immediately  after  sulfuring  and  at  no  time  exposed  to  the  direct  rays 
of  the  sun.  The  drying  time  in  this  case  was  eleven  days.  The  yields 
and  the  sugar  contents  of  the  two  products  as  given  at  the  bottom 
of  Table  IV   are  practically  identical.     The   stack-dried   fruit  was 


Fig.  1.     Small  air-blast  tunnel  dehydrater  in  Fruit  Products  Laboratory. 


Bulletin  330 


DEHYDRATION    OF   FRUITS 


67 


similar  in  color  to  the  dehydrated  peaches  in  that  unripe  fruit  re- 
mained green  in  color  after  drying  and  ripe  fruit  was  paler  yellow  in 
color  than  when  sun-dried.  The  stack-dried  peaches  were  cleaner 
in  appearance  and  slightly  better  in  flavor  than  the  sun-dried. 

Sulfur ous- Acid   Content   of  Sun-Dried  and  Dehydrated  Fruits: 
Samples   of   pears,    apricots,    and   seedless   raisins   which   had   been 


TABLE  V 

Sulfur  Dioxide  in  Sun-Dried  and  Dehydrated  Fruits 


Fruits  and  Hours 

method  of  drying  sulf  ured 

Pears 

University  dehydrater 1 

University  dehydrater 2 

University  dehydrater 4 

University  dehydrater 24 

Sun  2 

Sun  4 

Sun  and   shade 24+ 

University  dehydrater 3 

University  dehydrater 24+ 

Apricots 

University  dehydrater 

Sun  

University  dehydrater 

Sun  

Stack  evaporator 

University  dehydrater 

Sun  


Seedless  grapes   (Sultanina) 

University  dehydrater 

University  dehydrater 

University  dehydrater 

Stack   evaporator 

Stack   evaporator 


Sulfur  dioxide, 

parts  per 

million 


70 
191 
645 
632 
270 
536 
765 


Remarks 


344 
736 

236 
706 
1329 
1068 
937 
678 
702 

403 
910 
678 

408 
1188 


From  same  lot  of  pears 


Sample  from  trays- 
Lake  County 
Peeled  halves 
Average  for  season 


Comparative  lots 

Comparative  lots 
Average  sample 
Comparative  lots 


Exposed  to  sun  3  hours 


Exposed  to  sun  3  hours 
1000  parts  per  million  equals  0.1  per  cent. 


sulfured  various  lengths  of  time  and  dried  in  different  ways  were 
analyzed  for  sulfur  dioxide,  with  the  results  shown  in  Table  V.  The 
samples  had  not  been  subjected  to  the  resulfuring  commonly  given 
before  packing  and  therefore  contain  less  sulfur  dioxide  than  is 
normally  found  in  commercial  samples.  It  is  evident  that  the  amount 
of  sulfur  dioxide  absorbed  by  the  fruit  and  retained  after  drying 
increases  with  the  time  of  exposure  to  sulfur  fumes.    Since  dehydrated 


68  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

fruits  require  only  a  very  short  sulfuring,  in  general  not  over  one 
hour,  they  contain  much  less  sulfur  dioxide  than  similar  fruit  sul- 
fured  many  hours  preliminary  to  sun-drying.  Dehydrated  fruits 
come  from  the  dehydrater  clean  and  sterile  and,  if  properly  stored 
and  packed,  need  not  be  resulfured,  thereby  eliminating  any  objection 
on  the  part  of  the  consumer  because  of  ' '  sulf urous ' '  fruit. 

STEMMING 

The  stemming  of  dehydrated  grapes  gave  considerable  trouble 
.during  the  past  season  largely  because  of  insufficient  drying,  sweating 
of  dehydrated  grapes  in  bins  before  stemming,  or  improper  adjust- 
ment of  stemming  machines.  Unless  dehydrated  grapes  were  stemmed 
soon  after  drying,  the  grapes  and  stems  stuck  to  the  stemmer  and 
formed  masses  of  the  unstemmed  grapes.  Where  grapes  were  allowed 
to  stand  or  sweat  after  removal  from  the  dehydrater,  the  dry  brittle 
stems  reabsorbed  water  from  the  grapes  or  the  air  and  became  flexible, 
preventing  them  from  breaking  from  the  berries  readily.  The 
absorption  of  moisture  by  the  stems  also  caused  them  to  increase  in 
specific  gravity,  increasing  the  difficulty  of  fanning  out  the  stems. 
A  blast  strong  enough  to  remove  the  stems  caused  loss  of  small  dried 
grapes  in  the  stemmer  waste.  Insufficiently  dried  grapes  presented 
the  same  difficulties. 

The  dried  grapes  did  not  stem  satisfactorily  when  they  contained 
more  than  15  per  cent  of  moisture.  If  the  grapes  are  sticky  from  heavy 
steaming  or  dipping,  they  should  contain  only  12  per  cent  or  less  for 
satisfactory  stemming.  Grapes  dried  without  dipping  were  stemmed 
at  a  higher  moisture  content  than  steamed  or  dipped  grapes.  At  the 
University  Farm  in  1920,  the  grapes  were  dried  to  from  13  to  15  per 
cent  moisture  and  stemmed  as  soon  as  removed  from  the  dehydrater. 
With  careful  adjustment  of  the  stemmer,  excellent  results  were 
obtained. 

Cherries  may  be  stemmed  satisfactorily  by  machinery  before 
drying  but  the  stemming  is  best  done  after  drying.  Ordinary  raisin 
stemmers  can  probably  be  adjusted  for  this  purpose.  On  a  small  scale, 
the  stems  can  be  quickly  removed  by  rubbing  the  dried  cherries  on  a 
screen  tray  of  %  to  V2  inch  mesh.  The  brittle  stems  break  off  and 
drop  through  the  screen. 


Bulletin  330  DEHYDRATION  OF  FRUITS  69 


PROCESSING 

Laboratory  experiments  and  the  experience  of  several  commercial 
plants  demonstrated  that  steaming  of  stemmed  grapes  affords  a  simple 
and  effective  means  of  returning  to  the  dried  product  the  moisture 
necessary  to  replace  that  removed  to  make  stemming  possible.  This 
represents  the  difference  between  about  14  and  22  per  cent,  or  a  total 
of  8  per  cent. 

A  very  satisfactory  system  of  processing  dried  grapes  developed 
in  one  plant,  consisted  in  scraping  the  trays  into  a  hopper  from  which 
a  conveyor  carried  the  grapes  to  the  stemmer.  The  stemmed  grapes 
discharged  on  to  a  second  conveyor,  which  carried  them  through  a 
long  narrow  box  filled  with  an  abundance  of  live  steam.  The  hot 
processed  grapes  discharged  directly  into  the  packing  box.  The  grapes 
absorbed  water  at  the  rate  of  3  to  4  per  cent  per  minute,  the  time  of 
processing  being  regulated  by  the  speed  of  the  conveyor.  The  steamed 
grapes  were  bright  and  clean  in  appearance.  Grapes  processed  in 
hot  water  lost  much  color  and  sugar  and  were  duller  in  color.  In 
some  plants,  the  desired  amount  of  water  was  added  to  the  piles  of 
stemmed  raisins  which  were  then  thoroughly  mixed  by  shoveling  in 
order  to  equalize  the  moisture.  This  method  is  neither  cheap  nor 
sanitary  and  does  not  sterilize  insect  eggs. 

The  processing  and  packing  of  prunes  gave  trouble  in  several 
packing  houses  in  1920.  When  deliveries  of  dehydrated  prunes  con- 
taining over  20  per  cent  of  moisture  were  mixed  with  sun-dried  prunes 
of  16  per  cent  or  less  moisture,  and  the  resulting  mixture  processed 
in  hot  water  in  the  usual  way,  the  tender  and  moister  dehydrated 
prunes  absorbed  so  much  water  that  they  not  only  tended  to  dis- 
integrate but  were  liable  to  mould  after  packing.  If  prunes  are 
not  over-dipped  nor  dried  at  such  a  high  temperature  as  to  cause 
cracking,  and  finally  are  dried  to  the  same  moisture  content  as  sun- 
dried  prunes,  they  may  be  readily  processed  and  packed  in  the  same 
manner  now  used  for  sun-dried  prunes.  It  is  always  best,  however, 
to  keep  dehydrated  prunes  separate  in  order  that  the  time  of  process- 
ing may  be  adjusted  to  their  moisture  content.  Since  prunes  as  well 
as  all  dehydrated  fruits  come  from  the  dehydrater  clean  and  sterile, 
it  should  not  be  necessary  to  over-dry  them  and  then  return  part  of 
the  moisture  by  dipping.  Where  it  is  necessary  to  store  the  fruit  for 
some  time  in  large  bins  for  blending  or  other  purposes,  steam  process- 
ing is  the  most  satisfactory  way  of  sterilizing  the  product. 


70  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


MOISTURE    CONTENT 

The  United  States  Department  of  Agriculture  has  placed  the  legal 
limit  for  moisture  in  dried  apples  at  24  per  cent.  It  is  likely  that 
similar  standards  will  in  time  be  adopted  for  all  dried  fruits.  When 
this  is  done,  accurate  control  of  moisture  in  the  finished  product  will 
become  a  necessity.  But  it  is  always  to  the  interest  of  the  producer 
to  market  his  product  with  the  maximum  amount  of  moisture  which 
will  permit  it  to  keep  indefinitely.  In  order  to  have  a  careful  check 
on  the  moisture  content,  it  is  highly  desirable  for  every  plant  to  make 
frequent  moisture  determinations  on  representative  samples.  In  order 
to  obtain  dependable  results,  it  is  of  primary  importance  to  sample 
fairly  large  quantities  after  thorough  mixing.  About  one  pound  of 
pitted  fruit  is  then  taken  and  ground  through  a  food  chopper. 

The  official  method  of  determining  moisture  in  dried  fruits  con- 
sists in  drying  a  10  gram  sample  for  exactly  12  hours  in  a  vacuum 
oven  at  158°  F.  and  at  a  vacuum  of  29  inches  mercury.  This  method 
necessitates  the  use  of  equipment  costing  several  hundred  dollars  and 
is  time-consuming.  A  modification  of  this  method,  used  in  the  Fruit 
Products  Laboratory,  consists  in  employing  a  temperature  of  200°  F. 
and  a  vacuum  of  29  inches  mercury  for  exactly  two  hours.  It  has 
been  shown  that  this  more  rapid  method  gives  results  sufficiently  close 
to  the  official  method  for  commercial  purposes. 

A  simpler  but  less  reliable  and  less  rapid  method  consists  in  drying 
a  10  gram  sample  for  a  specified  time  in  a  steam  or  hot  water  jacketed 
oven  at  212°  F.  The  times  usually  adopted  are  3%  hours  for  apricots, 
4V2  hours  for  peaches,  4  hours  for  apples,  5  hours  for  pears,  4  hours 
for  grapes,  and  3  hours  for  prunes.  The  loss  in  weight  in  each  instance 
multiplied  by  ten  represents  the  percentage  of  moisture. 

Experiments  in  progress  since  1919  show  that  dehydrated  grapes 
containing  not  more  than  23  per  cent  of  moisture  will  keep  indefinitely. 
Samples  containing  23  to  30  per  cent  of  moisture  sooner  or  later 
become  mouldy,  while  those  above  30  per  cent  soon  ferment  unless 
heavily  sulfured  to  prevent  spoiling. 

Prunes  containing  more  than  25  per  cent  of  moisture  in  most 
instances  become  mouldy. 

Moisture  standards  for  dried  peaches,  apricots,  and  pears  are  more 
difficult  to  determine  because  of  the  complication  caused  by  the  pre- 
servative effect  of  varying  concentrations  of  sulfurous  acid  in  these 
products. 


Bulletin    330  DEHYDRATION    OF   FRUITS  71 


COST   OF    DEHYDRATION 

The  costs  of  dehydration,  both  operating  costs  and  fixed  charges, 
are  discussed  in  a  report  of  this  Station  entitled,  "Some  Factors 
Affecting  Dehydrater  Efficiency."  The  operating  costs  for  the  de- 
hydration of  apricots,  peaches,  pears,  and  grapes  in  the  University 
Farm  dehydrater  are  given  in  Table  VI.  The  total  costs  given  in  this 
table  should  not  be  taken  as  typical  of  commercial  operation  because 

TABLE  VI 

Approximate  Operating  Cost  of  Dehydrating  Various  Fruits  in  the 
University  Farm  Dehydrater 

Apricots  Peaches  Pears  Grapes 

, A ^  f A ^  f A ^  ^ A, ^ 

Per  Per  Per  Per  Per  Per  Per  Per 

green  dry  green  dry  green  dry  green  dry 

Item  ton  ton  ton  ton  ton  ton  ton  ton 

Cutting,  20c  per  box  $8.00       $46.40       $7.51       $35.82       $6.78       $30.51        

Labor,  tray  men  and 
operator,  50e  per 
hour    3.66         21.23         3.40         16.22         4.26         19.17       $4.16       $14.56 

Fuel,  stove  oil,  8  c 
per  gallon  for  dry- 
ing and  dipping..     1.36  7.89         1.99  9.49         3.48         15.66         3.62         12.67 

Electricity,  power 
and  light,  3.7c  per 
k.w.  hour  30  1.74  .40  1.91  .50  2.25  .45  1.50 

Sulfur,  3.5c  per  lb.       .15  .87  .20  .96  .42  1.89  .05  .18 

Lye,  7c  per  lb 28  .98 

Total  operating 

cost $13.47       $78.13     $13.50       $64.40     $15.44       $69.48       $8.56       $29.89 

considerable  extra  expense,  principally  for  labor  and  fuel,  was  in- 
curred in  the  conduct  of  experiments.  On  the  other  hand,  the  salary 
of  a  superintendent  is  not  included,  his  duties  having  been  performed 
by  the  writers.  The  cost  of  dehydration  in  commercial  operations 
should  not  exceed  that  of  sun-drying,  since  the  extra  cost  of  fuel  and 
power  may  be  largely  offset  by  the  decreased  cost  of  labor  in  handling 
trays. 

Owing  to  the  fact  that  the  transformers  and  motor  used  at  the 
University  Farm  Dehydrater  were  not  large  enough  to  operate  the 
fan  at  the  desired  speed  and  also  because  of  the  undesirable  position 
of  the  fan  discharge,  the  air  flow  was  inadequate  for  most  economical 
operation.  These  defects  can  be  readily  remedied.  The  drying  time 
was  considerably  longer  than  if  ample  air  flow  had  been  available. 
This,  together  with  the  fact  that  the  dehydrater  was  not  always  oper- 


72  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

ated  at  full  capacity,  made  certain  costs,  such  as  fuel,  power,  and 
operator's  wages,  greater  than  would  otherwise  be  necessary. 

In  the  case  of  grapes,  about  one-fourth  of  the  fuel  consumed  was 
used  for  dipping  and  about  10  per  cent  of  the  power  cost  was  utilized 
in  operation  of  the  stemmer.  In  two  commercial  plants,  the  total 
operating  costs  for  the  dehydration  of  grapes  were  $12.83  and  $8.80 
per  green  ton,  respectively. 

The  cost  of  dehydration  of  prunes  as  obtained  in  four  commercial 
plants  is  given  in  Table  VII.  The  three  air-blast  dehydraters  referred 
to  are  all  of  fairly  satisfactory  design,  the  operating  costs  being 
approximately  equal.  The  greater  cost  of  fuel  in  the  stack  evaporator 
is  caused  by  lack  of  air  recirculation.  Recirculation  of  air  is  an 
essential  factor  in  fuel  economy.  The  greater  labor  cost  in  the  stack 
evaporator  is  necessitated  by  individual  handling  of  trays  as  com- 
pared to  the  truck  load  movement  of  trays  in  tunnel  dehydraters. 

The  cost  of  dehydrating  apples  in  one  large  plant  in  1920  was 
given  as  $115  per  dry  ton,  which  included  $10  for  box  shook,  $5  for 
fuel,  and  $100  for  labor.  Since  the  charge  for  custom  drying  was 
$125  per  dry  ton,  this  would  leave  only  $10  per  dry  ton  for  fixed 
charges  and  profit.  In  another  plant,  the  labor  cost  was  shown  to 
be  $90,  the  fuel  $8.40,  and  the  power  $3.90  per  dry  ton. 

TABLE  VII 
Typical  Operating  Costs  in  Prune  Dehydraters 

Cost  per  green  ton 


Type  of  dehydrater  Labor  Fuel  Power  Total 

University   Farm   type $5.88  $1.27  $  .90  $8.05 

Air-blast  tunnel  5.30  2.10  1.00  8.40 

Air-blast  tunnel   6.00  1.65  1.40  9.05 

Stack  type,  natural  draft 12.53  3.29  .10  15.92 


MISCELLANEOUS    FRUITS 

In  addition  to  the  investigations  already  reported,  less  extensive 
experiments  have  been  made  in  the  dehydration  of  figs,  berries,  olives, 
persimmons,  and  citrus  fruits. 

Figs:  In  1919,  about  seventy-five  pounds  of  Mission  figs  were  dried 
in  the  sun  at  the  University  Farm  and  two  lots  of  similar  fruit  were 
dehydrated.  Of  the  latter,  one  lot  was  dipped  in  dilute,  boiling  lye 
solution  and  rinsed  in  water,  while  the  other  was  untreated.  They 
were  dehydrated  at  not  above  150°  F.  Both  of  the  dehydrated  lots 
were  slightly  over-dried,  as  they  were  placed  on  a  car  with  grapes, 


Bulletin  330  DEHYDRATION   OF  FRUITS  73 

which  require  a  longer  drying  period.    The  drying  time  was  less  than 
fifteen  hours,  the  dipped  fruit  drying  more  rapidly  than  the  undipped. 

The  surface  of  the  dehydrated  fruit  was  glossy  after  drying,  par- 
ticularly that  which  had  been  lye  dipped.  Both  dehydrated  lots  were 
satisfactory  in  every  respect  and  superior  in  flavor  and  cooking  quality 
to  the  sun-dried  product. 

"Calimyrna"  figs  from  Merced  were  received  at  the  University 
Farm  dehydrater  in  1920.  The  fruit  was  well  ripened  but  some  .of  it 
had  begun  to  sour  in  transit.  It  was  divided  into  three  lots.  One 
was  dehydrated  untreated  on  screen  trays ;  another,  dehydrated  after 
sulfuring  for  six  hours  on  slat  trays ;  and  the  figs  of  the  third  lot  were 
slit  down  one  side,  as  in  the  packing  of  dried  figs  in  brick  form,  and 
spread  apart  upon  trays.  Nine  hours  at  165°  F.  was  required  to  dry 
the  whole  fruit.  The  slit  fruit  dried  much  more  rapidly.  No  notice- 
able injury  to  color  or  flavor  occurred  in  drying  at  165°  F.  The  slit 
fruit  presented  a  glossy  surface  and  attractive  appearance.  The  sul- 
fured  figs  were  slightly  lighter  in  color  but  were  not  quite  so  attractive 
in  flavor  as  the  unsulfured.  The  "sour"  or  fermented  flavor,  which 
was  noticeable  in  a  great  deal  of  the  fresh  fruit,  completely  dis- 
appeared during  dehydration,  a  fact  which  should  interest  those  who 
grow  white  figs  in  sections  where  souring  and  splitting  occur.  These 
preliminary  experiments  indicate  the  possibility  of  fig  dehydration, 
but  further  investigation  is  needed  to  establish  proper  procedures, 
costs,  etc. 

'  Strawberries:  Several  of  the  important  commercial  varieties  of 
strawberries  were  dehydrated  experimentally.  The  fruit  was  shipped 
direct  from  the  grower  to  the  dehydrater  within  less  than  24  hours 
after  picking. 

At  temperatures  above  130°  F.,  the  fresh  fruit  "leaked"  badly, 
losing  a  great  deal  of  its  juice.  The  best  results  were  obtained  by 
starting  drying  at  110°  F.  and  progressively  increasing  the  tempera- 
ture to  160°  F.  Twenty-six  to  twenty-eight  hours'  drying  time  was 
required  under  these  conditions.  The  finished  product  was  of  un- 
attractive appearance  but  of  excellent  flavor.  The  dehydrated  berries 
were  very  satisfactory  for  use  in  preparing  preserves,  jams  and  pies. 
Sulfuring  for  one  half  hour  before  dehydration  improved  the  color. 
Halved  berries  dried  more  rapidly  and  were  more  attractive  in  appear- 
ance than  the  whole  fruit,  but  the  cost  of  such  a  method  of  preparation 
would  be  excessive. 

Loganberries:  Firm,  ripe,  freshly  picked  loganberries  yielded  a 
dehydrated  product  of  excellent  flavor  and  color,  with  the  individual 
berries  equal  in  size  to  the  original  fresh  fruit.    Over-ripe  and  bruised 


74  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

berries  lost  considerable  juice  by  dripping  and  tended  to  form  * '  slabs. ' ' 
The  parallel  current  system  was  used  successfully.  The  drying  was 
started  at  200°  F.  and  6  per  cent  relative  humidity,  and  was  completed 
at  160°  F.  and  30  per  cent  relative  humidity.  Seven  to  eight  hours' 
drying  time  was  required  with  the  above  temperature  range  and  an  air 
velocity  of  975  feet  per  minute. 

Raspberries:  Raspberries  were  dehydrated  without  preliminary 
treatment.  A  dehydrated  product  of  excellent  color  and  flavor  was 
obtained  by  starting  drying  at  189°  F.  and  10  per  cent  relative 
humidity  and  finishing  at  160°  F.  Using  air  at  a  velocity  of  975  feet 
per  minute,  drying  was  completed  in  4%  hours. 

Olives:  Pickled  ripe  olives,  dehydrated  at  350°  F.  or  above,  re- 
mained plump  and  equal  in  size  to  the  original  fruit.  The  flavor  of 
the  dehydrated  product  was  pleasing,  although,  when  stored  in  the 
open  air,  the  fruit  became  rancid.  Storage  in  vacuum-sealed  con- 
tainers would  probably  overcome  this  difficulty. 

When  dried  at  temperatures  of  160°  F.  to  250°  F.,  the  fruit  was 
badly  shriveled,  although  superior  in  flavor  to  that  dehydrated  at 
350°  F. 

Shredded  ripe  olives  were  dried  in  3  hours  at  210°  F.  to  220°  F. 
The  product  was  of  rich  flavor  and  suitable  for  cooking  in  macaroni, 
spaghetti,  meat  pies,  and  many  other  dishes.  The  drying  ratio  of  the 
shredded  olive  flesh  was  3  :1.  It  is  possible  that  small  sizes  of  olives 
might  be  utilized  in  this  way. 

Persimmons :  Persimmons  are  dried  in  large  quantities  in  Japan 
and  China  by  placing  the  peeled,  ripe  fruit  on  strings  in  the  shade. 
The  finished  product  is  brown,  soft,  sticky,  and  very  sweet.  It  is  free 
from  the  "puckery  taste"  of  unripe  persimmons. 

Attempts  to  dehydrate  peeled,  whole,  ripe  fruit  proved  that  an 
excessively  long  drying  period  (more  than  48  hours  at  150°  F.  to 
165°  F.)  was  necessary.  Excellent  results  were  obtained  by  peeling 
and  slicing  the  firm,  ripe  fruit  and  drying  it  without  further  treat- 
ment. Even  very  astringent  fruit  became  sweet  and  free  from 
"pucker"  when  dried  in  this  manner.  Sulfuring  the  sliced  fruit, 
even  for  so  short  a  period  as  fifteen  minutes,  gave  an  intensely 
astringent,  dried  product  and  did  not  materially  improve  the  color. 

The  dehydrated  fruit  refreshed  well  on  soaking  in  water.  It  also 
made  a  pleasing  confection  when  eaten  in  the  dry  state.  The  yield 
was  approximately  one  pound  of  dry  to  three  pounds  of  fresh  fruit. 

Bananas:  In  the  tropics,  bananas  are  usually  dried  in  the  sun  after 
peeling.    Such  dried  fruit  is  dark  brown  and  of  unattractive  appear- 


Bulletin  330 


DEHYDRATION    OF   FRUITS 


75 


ance.  It  is  marketed  under  the  name  of  "banana  figs."  A  dried 
product  of  higher  quality  is  produced  commercially  by  artificial  heat, 
although  the  industry  is  still  small.  Large  quantities  of  bananas  go 
to  waste  in  banana-exporting  countries.  Dehydration  would  afford 
a  simple  means  of  preventing  this  waste  by  putting  the  fruit  in  a 
form  suitable  for  export  to  other  countries.  Experiments  in  the  Fruit 
Products  Laboratory  demonstrated  that  a  dried  product  of  attractive 
appearance  and  pleasing  flavor  could  be  made  from  bananas  in  the 


'  M                        is             3            ■    m  vm      a  'J9 

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Fig.  2. — Upper  left:  Cooperative  dehydrater  built  by  Farm  Bureau  at  Placer- 
ville.  Lower  left:  Building  a  University  Farm  dehydrater  at  Los  Gatos.  Eight: 
Unloading  trays  at  University  Farm  dehydrater,  Davis. 


following  manner.  The  ripe  fruit  was  peeled  and  sliced  longitudinally 
in  halves  or  in  strips  about  a  quarter  of  an  inch  in  thickness.  It  was 
then  spread  on  trays,  sulfured  for  twenty  to  thirty  minutes,  and 
dehydrated.  A  temperature  of  200°  F.  was  used  while  the  fruit  still 
contained  a  large  amount  of  water,  but  165°  F.  should  not  be  exceeded 
during  the  last  stages  of  drying.  Twenty  per  cent  moisture  in  the 
finished  product  gave  a  very  desirable  texture.  The  drying  ratio  of 
the  peeled  fruit  was  approximately  3  :1 ;  of  the  impeded  fruit,  4.5  :1. 
The  fruit  in  the  dry  state  was  satisfactory  for  confection  or  dessert 
purposes  or,  after  soaking  overnight  in  water  or  milk,  for  cakes, 
puddings  or  pies. 


76 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Citrus  Fruits:  Oranges  and  lemons  have  been  dried  upon  a  com- 
mercial scale  by  artificial  heat  without  other  treatment  than  slicing 
and  placing  on  trays.  The  fruit  has  been  dried  "bone  dry"  and 
ground  to  a  powder  and  used  in  bakeries  and  restaurants  for  pastry 
purposes. 

Orange  and  lemon  peels  are  also  dried,  either  in  the  sun  or  by 
artificial  heat,  for  the  preparation  of  extracts  and  flavors.  For  this 
purpose  only  the  outer  yellow  portion  of  the  peel  is  desired.  The 
market  is  limited. 

Our  experiments  demonstrate  that  a  good  lemonade  may  be  made 
from  dried,  sliced,  unpeeled  lemons,  A  short  sulfuring  before  drying 
gives  a  lighter  colored  product.  Dried  citrus  fruit  juices  in  powdered 
form  have  been  produced  in  spray  driers,  such  as  are  used  for  drying 
milk. 


Pounds  per 

Variety  of  sq.  ft.  on  Hours 

fruit  trays  sulfured 

Apples   2  y2 

Apricots    2  1 

Apricots    2  % 

Bananas    1-2  % 

Cherries 

Black  Tartarian      2-3  0 

Koyal  Anne  ....  2-3  % 

Figs   2-3  1 

Grapes 

Muscat   3%-4  0 

Seedless    3V2-4  1 

Wine  3%-4  1 

Loganberries 1%— 2  0 

Peaches   3  1 

Peaches  3  1 

Pears   3  24 

Pears   2  y2 

Pears  2  1 

Prunes 

French 2M>-4  0 

Imperial  3-4  0 

Raspberries  IMj-2  0 

Strawberries  1Mj-2  Mi 


TABLE  VIII 

for  Dehydration  of 

1  Various  Fruits 

Maximum 

temperature 

at  end  of 

drying 

period 

Desirable 

humidity 

in  tunnel 

dehydrater 

at  end  of 

drying 

period 

Drying 
time  by 
counter 
current 
method, 
hours 

Remarks 

165° 

F. 

5-10% 

8 

Peeled  and  sliced- 
or  cubed 

160° 

F. 

10% 

12 

Halves  unpeeled 

160° 

F. 

10% 

8 

Sliced 

165° 

F. 

5-10% 

12- 

-18 

Peeled,  cut  in  hal 
lengthwise 

170° 

F. 

10-25% 

8- 

-12 

Lye  dipped 

170° 

F. 

10-25% 

8- 

-12 

Lye  dipped 

160° 

F. 

5% 

10 

One  side  cut  and 
figs  spread  open 

160° 

F. 

5% 

24 

Lye  dipped 

160° 

F. 

5% 

16 

Lye  dipped 

160° 

F. 

5% 

20 

Lye  dipped 

160° 

F. 

10-25% 

10- 

-15 

Untreated 

150° 

F. 

10-20% 

24 

Not  peeled 

150° 

F. 

10-20% 

16 

Lye  peeled 

145° 

F. 

20% 

48 

Halves  unpeeled 

150° 

F. 

10-20% 

6 

Peeled  and  sliced 

150° 

F. 

10% 

16 

Peeled  and  cored 

165° 

F. 

5-10% 

1  il 

5-li 

21 

Lye  dipped 

165° 

F. 

10-20% 

30 

-36 

Lightly  dipped 

170° 

F. 

10-25% 

8- 

-12 

Untreated 

160° 

F. 

10-25% 

24 

Stemmed 

Bulletin  330  DEHYDRATION  OF  FRUITS  77 


SUMMARY 

The  results  reported  in  this  Bulletin  are  summarized  in  Table 
VIII,  where  are  listed  in  brief  form  the  tested  methods  of  preparation 
and  conditions  of  dehydration  recommended  for  various  fruits.  These 
recommendations  apply  to  the  air-blast  tunnel  type  of  dehydrater, 
which  so  far  has  proved  the  most  satisfactory  type  for  general  fruit 
dehydration. 

Further  investigations  in  the  dehydration  of  fruits  are  under  way 
and  many  operators  of  dehydraters  are  also  experimenting  on  various 
phases  of  dehydration.  It  is  fully  expected  therefore  that  many  of 
the  present  practices  of  dehydration  may  be  greatly  modified  during 
the  next  few  years.  Therefore,  it  may  be  necessary  to  revise  the 
recommendations  given  in  Table  VIII  as  our  knowledge  of  the  de- 
hydration of  fruit  increases. 

ACKNOWLEDGMENTS 

The  investigations  reported  in  this  bulletin  were  made  possible  by 
funds  from  the  appropriation  for  Deciduous  Fruit  Investigations 
made  by  the  state  legislature  of  1919. 

The  writers  are  indebted  to  the  many  manufacturers,  owners,  and 
operators  of  dehydraters  whose  generous  cooperation  made  possible 
the  securing  of  much  valuable  data.  Grateful  appreciation  is  extended 
to  the  California  Pear  Growers'  Association;  G.  H.  Hecke  of  Wood- 
land; E.  P.  Phillips  of  Winters;  Boyce  and  Boyce  Ranch,  Mr.  Wur- 
man, Manager,  of  Winters,  and  F.  W.  Yokum  of  Merced  for  much 
of  the  fruit  used  in  the  experiments  at  the  University  Farm.  Thanks 
are  also  due  to  the  Pacific  Wann  Evaporator  Co.,  and  the  Cutler  Dry 
Kiln  Co.,  for  the  use  of  equipment  installed  at  the  University  Farm. 
The  valuable  cooperation  of  G.  B.  Ridley  and  G.  R.  Kline,  of  the 
Heinemann-Pearson  Co.,  of  San  Francisco,  is  gratefully  acknowledged. 

The  writers  also  express  their  appreciation  to  Professor  F.  T. 
Bioletti  for  helpful  revision  of  the  manuscript. 


